<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <input type="text" value="" id="num1">
    <select name="" id="">
        <option value="+">+</option>
        <option value="-">-</option>
        <option value="*">*</option>
        <option value="/">/</option>
    </select>
    <input type="text" value="" id="num2">=
    <input type="text">
    <script>
        // 扩展案例（简单的计算器）
        
    </script>
</body>
</html>